Figure 9.

Working Model of ACHT1 in the Light.

Depending on environmental conditions, the photosynthetic reducing equivalents may be diverted from the productive reaction of (1) NADP⁺ reduction via ferredoxin (Fd) and ferredoxin NADP-reductase (FNR) to the nonproductive (2) Mehler reaction, producing hydrogen peroxide (H₂O₂) as an intermediate. A portion of the equivalents is used, (3) via ferredoxin Trx reductase (FTR), to reduce chloroplast Trxs, such as ACHT1, for regulatory purposes. The redox state of the chloroplast ACHT1 pool in the light is determined by its reduction rate by ferredoxin Trx reductase (3) and by its oxidation rate by 2-Cys Prx (Prx). 2-Cys Prx itself is oxidized by peroxides produced by nonproductive photosynthesis (2). ACHT1 reduction rate is limiting and leads to net oxidation of ACHT1 shortly after illumination. The redox state of ACHT1 is used for redox-based feedback regulation of the membranal electron transfer reactions.